A major goal in the projection television industry is to increase the brightness, contrast, uniformity at different viewing positions, and other aspects of the projected television image as it is viewed on a projection screen. This goal has prompted many research efforts, both in projection equipment and projection screens; but despite these efforts, significant improvement is still needed before broad and satisfying use of projection television can be expected.
At present, the screens most widely used for projection television are made from sheeting described in Chandler et al, U.S. Pat. No. 3,408,132, which comprises aluminum foil having a wrinkled surface prepared by compression rolling two sheets of the foil together under high pressure. This sheeting offers improvement in brightness of image over prior commercial beaded or lenticular projection screens, but it has a number of significant disadvantages--(a) an extreme susceptibility to damage, such that even a slight rubbing of the screen to clean it deforms the wrinkled metal surface and leaves a lasting blemish (some reduction of this problem has recently been achieved with oxide coatings applied to the metal foil surface); (b) a level of reflection that is too low to permit satisfactory viewing in lighted environments, such as a normally daylit-room; and (c) limitations in the angular range of reflection by the screen (because of such limitations in angular range, the wrinkled-foil screens are given a compound (horizontal and vertical) curvature to condense or aim the reflections at an audience; this compound curvature is achieved by adhesively mounting the sheeting on compound-curved substrates, which is an expensive, mistake-prone procedure; and even with the compound-curved screen the images projected by multi-tube color projection televisions, which typically comprise three side-by-side projection tubes each projecting a different color, take on a different hue or tint depending on the horizontal displacement of a viewer from the central projection axis).
Screens taught in Schudel, U.S. Pat. No. 4,089,587, were developed in response to some of the deficiencies of wrinkled-foil screens. The Schudel screens comprise a transparent polymeric film formed with minute vertical striations in one surface, a random matte texture in that or an opposite surface, and a layer of reflective material such as vapor-deposited aluminum on one of the surfaces. The film is adhered to a substrate, preferably with the metallized surface adhered against the substrate to provide durability.
The Schudel screens provide a very bright image to viewers who are close to the projection axis, and can be much more durable than the wrinkled-foil screens. Nevertheless, they have found only a limited acceptance, primarily because of the limited angular range of reflection from the screens. Outside a range of about plus-or-minus 5 degrees of the projection axis, the brightness of the image declines very rapidly.
A different approach not yet brought to commercial use is described in German Pat. No. 2,655,527. This approach relies on an oriented film of foamed polypropylene, such as described in Bottomley, U.S. Pat. No. 3,214,234. The orientation or stretching process leaves the exterior surface of the film densely packed with a random pattern of generally parallel elongated microscopic projections or recesses. The German patent teaches that when this surface is metallized, i.e., provided with a specularly reflective coating of vapor-deposited metal, it may be used as a projection screen for projection television and the like. The patent also suggests that a foamed film may be used as a master, for example, by forming a metal copy of the surface and using the metal copy to stamp the foamed surface into a plastic film, which itself can then be stretched and metallized.
We have tested metallized oriented foamed polypropylene projection screens as described in the German patent and find that they have good brightness; but there is an undesirable variation in brightness as a viewer moves from a position directly behind the projector. For example, by our measurements, such screens have a gain factor.sup.1 of 39 at a 0.degree. divergence angle, but the gain factor is only 29, or 67 percent of its 0.degree. value, at a horizontal divergence angle of 10.degree.. The gain factor further declines to 45 percent of the 0.degree. value at 20.degree. horizontal divergence, and to 29 percent at 30.degree. horizontal divergence. At the same time, the screen has a very narrow vertical distribution of light, with a gain factor at a vertical divergence angle of 5.degree. of only 6 percent of its 0.degree. value. The extremely narrow vertical distribution is a major drawback, since for viewers to see a projected image on sheeting with such a distribution would require that they be in a single row and all have the same eye level. FNT 1 Gain factor is the ratio of the reflection from a screen under consideration and the reflection from a standard white diffusing surface under the same illumination, and is stated as the number of times that the screen is brighter than the standard white diffusing surface. The gain factors reported herein were measured on a goniophotometer made by Gardner Laboratory, Inc., Bethesda, Md., Model No. GG 9204. With this instrument, light is beamed at the screen being tested on a line that is horizontally spaced about 15.degree. from a line perpendicular to the screen. Measurements are read with a sensor placed on the side of the perpendicular line opposite from the projected beam. When the sensor is at a position horizontally spaced 15.degree. on the side of the perpendicular line opposite from the projected beam (thus measuring the reflection at an angle equal and opposite to the angle of incidence), the measurement is regarded as the reflection at a 0.degree. divergence angle. This reading should be the peak reflection, since, as in specular reflection, the reflection is greatest when the angle of reflection is equal and opposite from the angle of incidence. When the sensor is positioned 25.degree. on the side of the perpendicular line opposite from the projected beam, the reflection measured is regarded as the reflection at a 10.degree. divergence angle; the 35.degree. reading is the reflection at a 20.degree. divergence angle; etc.
Prior to the commercial introduction of projection television, other attempts had been made to provide projection screens that were brighter than lenticular or beaded screens, which typically exhibit gain factors of less than 5. Burton, U.S. Pat. No. 2,660,927, describes a screen made from a clear piece of transparent material such as glass, the back of which is formed with a sinusoidal pattern of closely spaced vertical ribs; the back ribbed surface of the glass is mirror-plated, and the front surface is roughened sufficiently to remove glare. Disadvantages with this screen include the fact that a surface of regular configuration often produces scintillation, i.e., a flash of brilliant reflection from the screen seen at certain angles, which disrupts viewing of a projected image, and glass is heavy and fragile.
Sherwood, U.S. Pat. No. 3,492,060, teaches a screen offered as an improvement over the Burton screen, and which comprises a translucent sheet spaced in front of a reflective rear sheet corrugated with alternate ridges and grooves. This screen also suffers from scintillation, and in addition the spaced construction makes the product difficult to manufacture and apply to a substrate.
Mihalakis, U.S. Pat. No. 2,984,152, and Mihalakis et al, U.S. Pat. No. 3,063,339, also describe screens having a regular pattern of curved reflective surfaces. In the '152 patent, the reflective surfaces are vertical corrugations that undulate along their vertical length. The '339 patent teaches a more generalized surface designed to provide a desired field of observation; and a diffusing coating is applied over the configured surface. The '152 patent states that screens as described can be manufactured by preparing by hand a larger-than-scale model, and then preparing an embossing master by copying the model in reduced scale "on a three dimensional engraving machine of conventional design employing the pantograph principle." So far as known, no commercial sheeting of this type has ever appeared and in any event the sheeting would presumably suffer from scintillation.